So far, the post-plating method wherein a drawn container produced from cold rolled steel strip is plated in a barrel plating or pre-plating method, where a nickel plated steel strip is drawn into a battery container, have been employed for battery containers used for primary batteries such as alkali manganese batteries, secondary batteries such as nickel cadmium batteries, and a nickel-hydrogen battery in which a strong basic solution is packed, that is expected to be increasingly in demand as a new secondary battery. The reasons why nickel plating is employed for battery containers such as those used for alkali manganese battery or nickel cadmium battery are as follows:
1) a strong basic solution of potassium hydroxide is used as an electrolyte in these batteries, and nickel has excellent corrosion resistance to alkaline solutions. PA1 2) nickel has stable contact resistance when a battery is connected to an external terminal. PA1 3) spot welding is practiced when component parts are welded and assembled into batteries in the battery manufacturing process or when batteries are serially connected in order to increase voltage or when they are connected in parallel in order to allow large current flow, and nickel has excellent weldability. PA1 1) a nickel-tin alloy layer is formed as the uppermost layer on the surface that is to become the inner surface of a battery container; PA1 2) a nickel-tin alloy layer as the uppermost layer and a nickel layer as the lower layer are formed on the surface that is to become the inner surface of a battery container; PA1 3) a nickel-tin alloy layer as the uppermost layer, a nickel layer as the intermediate layer and a nickel-iron alloy layer as the lowermost layer are formed on the surface that is to become the inner surface of a battery container; PA1 4) a nickel-tin alloy layer as the uppermost layer and a nickel-iron alloy layer as the lower layer are formed on the surface that is to become the inner surface of a battery container; PA1 5) a nickel-tin alloy layer as the uppermost layer, an iron-nickel-tin alloy layer as the intermediate layer and a nickel-iron alloy layer as the lowermost layer are formed on the surface that is to become the inner surface of a battery container; PA1 6) a nickel-tin alloy layer as the uppermost layer, a nickel layer as the intermediate layer and a nickel-iron alloy layer as the lowermost layer are formed on the surface that is to become the outer surface of a battery container; PA1 7) a nickel-tin alloy layer as the uppermost layer and a nickel layer as the lower layer are formed on the surface that is to become the outer surface of a battery container; and PA1 8) a nickel layer is formed as the uppermost layer on the surface that is to become the outer surface of a battery container;
However, barrel plating causes unstable quality due to insufficient plating thickness and the difficulty of uniform deposition caused by insufficient circulation of plating solution deep into the bottom portion of the battery container when the inside of a tall cylindrical battery container is plated by barrel plating. On the other hand, although the above-mentioned problems are not caused in the pre-plating method, the battery container produced from a nickel-plated steel sheet, treated by thermal diffusion has improved corrosion resistance because the nickel plating layer is recrystallized and softened and thus has good extensibility. However, it has poor adhesion to the positive electrode mix because the inner surface of the positive electrode container (the battery container of the present invention) has small cracks and a smooth surface after drawing.
Thereupon, battery performance has a close relationship to the properties of the inner surface of the positive electrode container (the battery container of the present invention) in an alkali manganese battery (see FIG. 2). The better the adhesion of the positive electrode mix (composed of manganese dioxide as the positive electrode active material, graphite as the conducting material and potassium hydroxide as the electrolyte) of the alkali manganese battery to the inner surface of the battery container, the better the battery performance. In the case of an alkali manganese battery, the positive electrode mix is in contact with the battery container and the battery container functions not only as a container but also as an electrical conductor that transmits electrons. Therefore, when the contact resistance between the positive electrode mix and the inner surface of the battery container is large, the internal resistance of the battery is likewise large, and battery performance is deteriorated by the resultant drop of current or reduction of discharge duration. Therefore, it is preferable to reduce the contact resistance between the positive electrode mix and the inner surface of the battery container as little as possible in order to obtain a high performance battery.
Alkali manganese batteries are superior to manganese batteries in performance in high load electrical discharge where there is an especially large current flow, and the battery performance of the alkali manganese battery can be improved by reducing internal resistance of the battery. For the purpose of reducing the contact resistance between the positive electrode mix and the battery container to enable a large current flow, several methods such as roughening the inner surface of the battery container, providing grooves on the inner surface of the battery container in a lengthwise direction, and coating a conductive material composed of graphite added by binder on the inner surface of the battery container etc., are proposed. (See Battery Handbook, page 84, issued by MARUZEN in 1990)
Improvement in the contact between the positive electrode mix and the battery container causes a reduction of internal resistance, and consequently larger battery capacity can be obtained by reducing the amount of graphite in the positive electrode mix and increasing the amount of manganese dioxide as the positive electrode active material. Thus, battery performance depends considerably on the improvement of the internal resistance and particularly, the contact between the battery container and the positive electrode mix.
However, the use of a roughened punch in order to roughen the inner surface of the battery container causes the problem where the rougher the punch, the lower the drawability, and the punch can not be roughened beyond a certain extent.
Also, the use of a steel substrate having larger crystal grains to roughen the inner surface of the battery container after drawing causes the problem that the larger crystal grains result in a roughened surface at the positive electrode terminal and a deteriorated appearance for the battery container product in the case of a recently dominant pip type battery (the part of the positive electrode terminal of the battery container is convexly shaped).
Further, although a conductive paint coating or conductive material coating on the inner surface of the battery container can reduce internal resistance, it also causes disadvantages such as an increase in the process of the battery manufacturing and an increase in production cost.
Therefore, a battery material having a low cost of manufacture and low internal resistance is required for high performance alkali manganese batteries.